The U.N.-sponsored Syrian peace talks in Geneva were plunged into crisis Monday after opposition negotiators announced they are delaying involvement in the formal negotiations until the Assad government agrees to discuss the establishment of a transitional administration in Damascus.

Their announcement came as the U.S. and Russian brokered “cessation of hostilities” agreement appeared to be fraying fast, with both the government and rebels violating the truce. The Western-backed Free Syrian Army and other rebel groups issued a statement saying they had set up a joint operations room to respond to the violations by government forces and allied Iranian, Lebanese and Iraqi Shi’ite militiamen.

As fighting intensified in the Aleppo countryside, international humanitarian organizations warned that the already desperate plight of an estimated 100,000 refugees trapped in northern Syria will worsen. For months, the Turkish government has blocked Syrians fleeing the war from crossing the border.

Médecins Sans Frontières (MSF) (Doctors Without Borders) warned that renewed fighting Monday was “just seven kilometers” from where the bulk of the refugees have gathered.

Rebel negotiator Mohammed Alloush warned the truce “has effectively been ended by the regime.” He said the Assad government along with Russian warplanes carried out 70 airstrikes on Sunday, and he alleged that Iran had sent more troops to reinforce Assad’s forces. “All this intervention gives a clear indication that the solution in Syria, with the presence of this regime, has become shut — or we have hit a wall," he told the Associated Press.

U.N. envoy Staffan de Mistura sought to play down the significance of the opposition’s decision, saying the Geneva peace talks, now in their third round, will continue despite the rebels postponing their participation. De Mistura said he will continue “technical” discussions with all sides by phone in hopes of securing some agreement on the broad outlines of a political transition in Syria.

U.N. officials told VOA there are deep divisions among Syrian opposition officials, with some not happy about the decision to suspend participation and others supporting a withdrawal from Geneva until the cessation of hostilities is restored.

Riyad Hijab, the head of the rebel High negotiations Committee (HNC), was due to hold a news conference Monday night, but postponed it until Tuesday — a sign of disagreements within the opposition camp. Rebel commanders also are putting pressure on the HNC, says an HNC official. Nearly a dozen Free Syrian Army militias have warned of the need to “take firm and decisive stances towards the half-solutions being propagated … by the regime’s allies and de Mistura.”

The intensified fighting in the days leading up to the third round of the talks — the opposing sides have not even met face-to-face — augured badly, even before the negotiation teams arrived in the Swiss city. De Mistura said the new fighting in parts of Syria, especially around Aleppo, where the regime and its Iranian and Hezbollah allies as well as the Russians appear to be on the brink of launching a major offensive, was “particularly worrisome.”

The Assad government has made little secret of planning for an offensive on the rebel-held parts of Aleppo city. On April 10, Syrian Prime Minister Wael Nader al-Halqi told Russian news networks the government was preparing a major operation to retake the rebel controlled city districts.

“We, together with our Russian partners, are preparing for an operation to liberate Aleppo," he said.

Meanwhile, rebel factions, along with al-Qaida affiliate Jabhat al-Nusra, have been pressing an offensive in the coastal province of Latakia, the stronghold of Assad’s minority Alawite sect, an offshoot of Shi'ite Islam. They have insisted that the offensive was in retaliation for government bombing of civilians targets elsewhere in the country.

Syria's U.N. ambassador, Bashar Ja'afari, accused rebel fighters of calling for a “revocation of the cessation of hostilities.” Syrian opposition negotiator Mohammed al-Abboud countered by arguing that the rebels have the right to defend themselves and Syrian civilians.

“We have the right to retaliate and defend ourselves in case we are attacked and we will not be silent about any aggression and we will continue our main mission to defend civilians,” al-Abboud said on Twitter.